This invention relates to an electronic circuit for detecting a change with respect to a quiescent condition, in accordance with the preambles of claims 1 and 2.
U.S. Pat. No. 4,574,936 discloses such an electronic circuit in the form of a coin-operated vending machine. For comparing two coins, there is provided an inductive null detector, comprising a symmetric dual feedback oscillator, which is unbalanced when an inserted coin is different from a test coin. The field of application described in this patent specification is limited to comparing coins.
For measuring presence, position or movement of an object, it is known to make use of capacitive sensors, inductive sensors, strain gauges and Hall effect sensors.
For capacitive sensors, it holds that with increasing distance to the electronic measuring circuit having therein the capacitance to be influenced, the applicability decreases as a result of the presence of parasitic capacitance, possibly varying in time.
Existing inductive sensors and inductive proximity switches utilize electrical oscillators, coils, ferrite cores, and the like, while the target material is typically iron, steel, copper or aluminum, and for each target material a different sensitivity applies.
Hall effect sensors are frequently employed, and primarily measure a change of the local magnetic field, which change may be caused by relative movement of the sensor with respect to one or more magnets or other objects.
A common drawback of all these known sensors is that they are suitable only for specific fields of application and often require a complex detection circuit.
There is a need for a simple electronic circuit which can be adapted in a simple manner for a variety of applications which are based on the occurrence of a change in a quiescent condition. To be considered as applications are, for instance, the measurement of a position, of speed, of acceleration of an object, but also layer thickness measurement, determining material properties, hair crack detection, etc. An application of determining a position is the use as analog or digital proximity switch.